This project is designed to determine the relationships among DNA repair, chromosome structure, and mutagenesis in Drosophila melanogaster. Mutations that increase the mutant frequency (mutators) have been identified in the mu2 gene and characterized. The mutators greatly reduce the efficacy of repair of gamma-ray-induced chromosome breaks in oocytes, thereby allowing a previously undescribed repair pathway to be observed. By this newly identified repair pathway broken chromosome ends are "capped" with a new telomere. Mutator mutations appear to disrupt chromatin structure in the oocyte, but not the sperm chromosomes, so that oocyte chromosomes are not repaired properly. Given that mu2 mutations also affect mitotic recombination, it is reasonable to expect that the MU2 protein would associate with somatic cell chromosomes. This is confirmed by the observation that mu2 mutations also enhance variegation, that is they appear to enhance the formation of heterochromatin. We plan to look for other gene products that might interact with MU2 in chromatin. The mutator gene has been mapped using a nested set of 150 deletions to a region of 5 kilobasepairs. A candidate cDNA homologous to this region has been cloned and sequenced. The sequence does not have a strong resemblance to any other genes in the database. The genomic region including the transcription unit will be reintroduced into the genome to ascertain complementation with mu2 mutations.